Devices and methods are known for deposition of biological and non-biological materials in a variety of applications. Some such materials are nanofibers, which may have applications in medicine including artificial organ components, tissue engineering, implant material, drug delivery, wound dressing, and medical textile materials. For example, in wound healing applications, nanofibers, which have a large ratio of surface area to volume (i.e., a high aspect ratio), assemble at an injury site and stay in place thereby providing a close fit for direct and optimum material-tissue interaction, attracting and interacting with surrounding cells, and directing their development so as to lead to tissue regeneration. In addition, the fibers may be used to deliver biomolecules (for example, growth factors) to stimulate surrounding cells.
Nanofibers also may be used as protective materials such as sound absorption materials, protective clothing for use against chemical and biological warfare agents, and sensor applications for detecting chemical agents.
Further, nanofibers may be used in energy applications such as lithium-ion batteries, photovoltaic cells, membrane fuel cells, and dye-sensitized solar cells and antennas (e.g., high aspect ratio polymer fibers with carbon nanotubes).
Current methods for forming and depositing nanofibers include solution blow spinning, melt spinning, electrospinning, and melt blowing. Solution blow spinning typically involves a first nozzle that supplies a polymer solution (i.e., polymer and solvent) and a second nozzle that supplies a high pressure gas. The polymer solution is forced from the first nozzle and forms a drop or blob. The high pressure gas exiting the second nozzle stretches the polymer solution into a conical shape. When a critical pressure is reached, the polymer/gas mixture travels from the tip of the cone to a target, and as it does, the solvent evaporates leaving polymer fibers, which deposit on the target.
Melt spinning involves drawing down extruded strands of melted polymer to reduce fiber diameter and induce orientation of polymer chains. Melt spinning, however, is restricted to viscoelastic materials that can withstand the stresses developed during the drawing process. Fibers made by melt spinning typically are greater than 2 μm in diameter. A variation of melt spinning that produces nanofibers involves the production of several individual strands of one polymer component within a larger single strand of a second polymer component.
Electrospinning is similar to solution blow spinning in the aspect of forming a cone that stretches to produce polymer fibers. The motive force in electrospinning results from an applied voltage differential between the polymer output device (nozzle) and the target. Accordingly, electrospinning cannot be used on humans without risking harm to the target. Electrospinning has a low fiber production efficiency and is further limited because solvents compatible with the electrospinning process are limited by their dielectric constant.
Melt blowing involves extruding molten polymer through a narrow orifice and into a stream of high velocity hot air. The drag of the hot air on the surface of the molten polymer causes the polymer to elongate into a fiber. However, melt blowing cannot produce fibers with diameters in the same nanoscale size range as in other methods and further is limited to use of thermoplastic polymers.